newvers.sh: Take the src/ dir as an argument instead of hard coding it.

This fixes the issue where the last number in our version was included
only if /usr/src was present at build time, which resulted in different
output of uname -r, sysctl kern.{osrelease,version}, etc. across
different systems (e.g. "2.3.1-DEVELOPMENT" vs. "2.3-DEVELOPMENT").

This commit also removes the passing of ${KERN_IDENT} and ${IDENT} to
newvers.sh (both of which are not used by it) and performs some
additional cleanup.